Evolution of Hair - Evolution - Human Hairlessness

Human Hairlessness

The hairlessness of humans compared to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago. Mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.

In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation. Additionally, it can be used as a communication tool and as a camouflage. To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.

Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and other cetaceans, and naked mole rats. Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.

It was expected that dating the split of the ancestral human louse into two species, the head louse and the pubic louse, would date the loss of body hair in human ancestors. However, it turned out that the human pubic louse does not descend from the ancestral human louse, but from the gorilla louse, diverging 3.3 million years ago. This suggests that humans had lost body hair (but retained head hair) and developed thick pubic hair prior to this date, were living in or close to the forest where gorillas lived, and acquired pubic lice from butchering gorillas or sleeping in their nests. The evolution of the body louse from the head louse, on the other hand, places the date of clothing much later, some 100,000 years ago.

Balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals did not survive to adulthood, let alone reaching their fourth decade and therefore balding tends to act as a signal of maturity. In women survival to such an advanced age is usually coupled with a decrease in a fertility (see menopause), but in men fertility is retained beyond middle-age. The persistence (but non-ubiquity) of balding in men, coupled with its general absence in women, suggests that there was a selection pressure against balding in women (perhaps in the form of a pressure against signals of advancing age), but variations in hair patterns among men did not prevent their reproductive success leading to stable polymorphisms (perhaps representing different mating strategies); for example some men could have benefitted from baldness by signalling advanced maturity and social status; while other men simulated the appearance of youth and vigor by retaining their hair.

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed a greater distribution of sweat glands that enabled them to perspire over most of the body. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over their bodies. Aside from the mammary glands that produce a specialized sweat called milk, most mammals just have apocrine sweat glands on their armpits and loin. The rest of their body is covered in eccrine glands. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body. It is unclear to what degree other primates sweat in response to heat, however.

The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the arrector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.

Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

Several hypotheses explained hairlessness as a thermoregulatory adaptation to hot and dry savanna. The most known thermoregulatory hypothesis in modern paleoanthropology was proposed by Peter Wheeler (1984, 1985). He suggests that a need for decreased body hair originated as a response to climate change that began approximately 3 million years ago. At this time, the earth entered a period of global cooling that had a dehumidifying effect on the main early human habitats in East and Central Africa. Lush, wooded forests gave way to dry, grassland savannah; because of this, early humans were required to travel farther in search of food and water. As early humans diverged from their chimpanzee-lineage, they also became omnivorous in order to maximize calorie intake, an important distinction in a nutrient-scarce environment. Prey, however, are moving targets, and though early humans changed the traditionally ape-like appearance of the australopithecines and adapted long, strong legs to facilitate sustained running, dense, hairy coats still posed a potentially fatal risk of causing overheating during the chase. It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun. Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. The rise in eccrine glands occurred on the genes that determine the fate of epidermal stem cells in human embryonic development.

Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time. This theory, however, has significant holdings in today's cultural norms. There is no evidence that sexual selection would proceed to such a drastic extent over a million years ago when a full, lush coat of hair would most likely indicate health and would therefore be more likely to be selected for, not against, and not all human populations today have sexual dimorphism in body hair.

A further hypothesis is that human hair was reduced in response to ectoparasites. The "ectoparasite" explanation of modern human nakedness is based on the principle that a hairless primate would harbor fewer parasites. When our ancestors adopted group-dwelling social arrangements roughly 1.8 mya, ectoparasite loads increased dramatically. Early humans became the only one of the 193 primate species to have fleas, which can be attributed to the close living arrangements of large groups of individuals. While primate species have communal sleeping arrangements, these groups are always on the move and thus are less likely to harbor ectoparasites. Because of this, selection pressure for early humans would favor decreasing body hair because those with thick coats would have more lethal-disease-carrying ectoparasites and would thereby have lower fitness. However, early humans were not able to compensate for the loss of warmth and protection provided by body hair with clothing, and no other mammal lost body hair to reduce parasite loads.

A final view is the naked love theory proposed by James Giles. According to this account, human hairlessness has its origin in the ancestral mother-infant relationship. In the naked love theory our hairlessness is ultimately the result of bipedalism. Because of bipedalism, ancestral infants lost the ability to grasp the mother’s fur with their feet, as do other primate infants. They thus could no longer hold onto the mother themselves. Early bipedal mothers were therefore under evolutionary pressure to carry their infants. Consequently, infants survived only if mothers had a strong desire to hold them. Because of the pleasure of skin-to-skin contact, especially in breast-feeding, the desire to hold the infant would have been stronger in less hair-covered mothers who passed their hairlessness onto their infants. These infants would have had a greater chance of survival than did hair-covered infants. The selection process here has been called maternal selection. This theory explains why females and children have less body hair than do adult males; for it was in the mother-infant relationship that hairlessness was first selected for. Although sexual selection probably played a role later (males would have preferred hairless females who resembled their hairless mothers), the theory is the only one that does not depend solely on sexual selection to explain hairlessness. It also explains why sexual selection for naked skin would have would have started in the first place.